We have demonstrated in our mouse model that microorganisms with antitumor activity contain a variety of antigenic components some of which enhance and some of which inhibit tumor growth. The ability to do one or the other depends on tumor load and relative time of exposure of the animal host to tumor and treatment. A double stranded RNA fraction from mycobacteria (H37Ra) can produce a significant inhibition of tumor growth when administered together with irradiated tumor cells from 1 to 4 weeks prior to initiation of tumor growth. P. acnes, administered alone intralesionally, inhibits tumor growth and if the tumor load is not too great can produce complete cures in about 22 per cent of the mice. When P. acnes is administered intralesionally to mice which are presensitized with mycobacterial RNA and irradiated tumor cells the inhibition of tumor growth is greatest. A fraction of P. acnes, at one fifth the nitrogen of intact cells, produced an inhibition equal to that seen with intact bacteria and, furthermore, was equally effective on intralesionally treated as well as distally growing tumors. BIBLIOGRAPHIC REFERENCES: Millman, I., Scott, A.W., Halbherr, T., Youmans, A.S., and Youmans, G.P. Mycobacterial ribonucleic acid: Comparison with mycobacterial cell wall fractions for regression of murine tumor growth. Infect. and Immun. 14: 929-933, 1976. Millman, I., Maguire, H.C., Pass (Kavanaugh), M., Youmans, A.S., and Youmans, G.P. Mycobacterial RNA. A comparison with intact Mycobacteria for suppression of murine tumor growth. J. Med. 7-249-261, 1976.